Biosphere

The surface of the Earth is almost spherical. It is 71% water, and the land is very irregular, with 65% of surface land mass being in the northern hemisphere.

The result is an uneven distribution of heat, and consequently irregular wind and precipitation patterns. The consequence of this are patterns of land category and vegetation type which are irregular. These can be reduced to 6-12 distinct biomes, depending on the system used.

When we talk about the total of all zones of the Earth where life is found, this is the biosphere.

The Biosphere

Nearly all life is found in the oceans (maximum depth of 11 km), and within about a kilometre beneath the Earth's surface. Life has adapted to survive in practically all places on Earth, whether at the poles, or in arid desert at the equator, high in the mountains, or underground. This thin veneer of the surface of the planet is the biosphere.

Technically, the biosphere is the sum of all ecosystems, and materially is a closed system: life does not normally go walk-about around the cosmos. However, life is totally dependent on the Sun's energy and light. And the Earth's surface benefits from heat created by nuclear decay of uranium, thorium, and a few other radioactive substances, in its interior.

Biomes are large geographical regions with common factors, which determine the type and range of species within it. The biomes are the following:

Biomes are divisions of the biosphere into regions with climatic conditions distinct enough to produce characteristics that are evident in vegetation types.

The Role of Climate

Why things live where they live is a complex question. A simple answer is: 'because they can'.

Life is found all over the planet. It is very determined, and adapts to the conditions in which it finds itself. The most obvious factor in determining these conditions is the climate. The latitude (how far north or south) is the most important of all climatic factors, since the intensity of the Sun's radiation varies most directly with latitude.

The Earth is a sphere, so the part of the Earth receiving the most vertical sunlight (the middle, or equatorial regions) will receive the most sunlight, and be hotter. As we move further north or south, the same amont of sunlight is spread over a larger area, and therefore the temperature and irradiation (sunlight available for photosynthesis) decreases (for those who like maths: by the sine of the angle of incidence).

Seasons are caused by the angle of tilt of the Earth, making the sun's heat inconsistent at any one point on the surface

The Earth is tilted to the plane of its orbit around the Sun. This means the line between the north and south poles (axis of rotation) is not at right angles to the line Earth-Sun (in the orbital plane). The angle of tilt is 23.44°. As the Earth goes around the Sun, the tilt causes the angle the sunlight strikes the Earth's surface at (angle of incidence) to change, and therefore the amount of sunlight that hits any place on Earth will vary continuously throughout the year. This causes seasons.

The hierarchy of living areas:

Biosphere: The entire life-supporting region of the Earth. The Biosphere is all the terrestrial surface, to a depth of about 1 km or more, the seas and waterbodies, and the troposphere.

Biome: a large planetary region with similar climatic and vegetation characteristics.

Ecosystem: a physically self-contained area and its communities, with largely intra-dependent foodchains and matter and energy flows.

Biota: the collective of living elements of an ecosystem.

Habitat: the set of conditions within a physically limited area, which are necessary for a specific species to survive.

Biomes

Biomes are divisions of the biosphere into regions with climatic conditions distinct enough to produce characteristics that are evident in vegetation and animal species adapted to them. Biomes may be variously defined, such as leaf type, vegetation density, climate, and patterns of succession

There are different ways of identifying biomes, but most systems include something close to these 6 major terrestrial biome types:

Tundra

The high latitudes, where sunlight is weak and winters harsh, with short daytimes, leads to a biome distinction in its lack of large vegetation, namely trees. The dominant vegetation is grass, moss, lichen, with the occasional shrub. Tundra has permafrost, which is ground that is frozen and never thaws. Less than 20% of the Tundra is inhabited by humans.

Tundra may be sub-divided into arctic, alpine and Antarctic tundras. As well as latitude, altitude creates ecological boundary regions (ecotones) in a treeline, at the changeover point between tundra and forest.

Taiga

Typical of northern Russia and Canada, the Taiga, or boreal forest, is a biome with coniferous forests (primarily larch, spruce, and pine).

Temperatures can be extremely low, and growth periods limited to less than 100 days a year. Trees are in competition for the low-intensity sunlight, so grow straight and tall, except where peat and water-logged terrain impedes growth.

Taiga soils are poor quality, due mainly to the cold. The Brown Bear is a dominant omnivore species.

Deciduous Forest

Those who live in northern Europe, and many other temperate zones, will be familiar with the deciduous forest: oak, elm, beech, maple attest a greater biodiversity than the Taiga.

Since deciduous trees lose their leaves in autumn, the canopy coverage seasonally changes dramatically. During canopy cover, species adapted to low-light conditions have adopted a wide range of survival strategies. Many tree species specialise in adverse conditions to out-compete more common species.

Rain Forest

The Rain Forest is a biome characterised by dense vegetation and extremely high biodiversity. Rainfall is high, and the monsoon is a critical factor in the development and maintenance of the tropical rainforest.

Rain forests can be tropical or temperate. As much as 60% of all species have their habitat in the rain forests. Rain forests provide services for the global environment, such as the carbon sink, and microorganisms for the pharmaceutical industry.

Rain forests are being depleted at an alarming rate, and as a result anthropological climate change is being accelerated, as well as being a major cause of the current biodiversity catastrophe.

Savanna

The Savanna, or grasslands, is a biome characterised by large swathes of open grassland, with scattered pockets of trees.

The density of trees can vary greatly in different savannas, but the open canopy permits richer ground-level ecosystems to develop.

The Savanna is often home to migratory herds of herbivores, such as the wildebeest and zebra in Tanzania. The migratory tendencies are due mainly to the seasonal precipitation patterns, and often there is a single 'wet season'.

Desert

Deserts are areas with a higher rate of evapotranspiration than precipitation. Deserts have less than 250mm of rainfall annually, although some deserts, such as in Chile, receive considerable amounts of moisture in the form of fog.

Semi-deserts, or steppes, have up to 500mm of precipitation, and may have grass covering.

Deserts are usually hot and sandy, however there are cold deserts, such as in polar regions.

Aquifers may bring water from mountain sources many hundreds of kilometres away. When these reach the surface, usually due to a fault in the subterranean rock strata, it may form an oasis, a micro-climate which forms an 'island' of thriving life in the middle of desolation.

Biotope

A biotope is similar to a habitat, but with a focus on the biological community (whereas habitat is focussed on population). A biotope describes an area in which uniform environmental conditions create a distinctive living space for a specific number of flora and fauna species in an interacting community.